1. Field of the Invention
The invention relates to a control method for a liquid-cooled cable installation with a hollow conductor, through which a coolant flows, as the cable conductor, the hollow space of which is divided in the longitudinal direction by partitions forming separate canals for the outgoing flow and the return of the coolant and in which canals the coolant is in contact with the conductor at high-voltage potential. Heat exchangers are provided at the start and the end of the cable system or at intermediate stations.
2. Description of the Prior Art
A liquid-cooled cable installation is disclosed in German Pat. No. 22 52 925. Water is used as the coolant there.
The use of high-tension d-c for the transmission of energy via cables has the substantial advantage in that the cable requires no charging power. The copper cross section of the cable can therefore be used fully for the transmission of the active current, especially because there is also no skin effect. A further, very important advantage over the use of three-phase current is the fact that a substantially higher field strength can be applied to the cable dielectric, i.e., a substantially smaller insulation thickness will be sufficient for the same voltage. For the same dimensions of a cable, a substantially larger current can be transmitted if d-c is used and, in addition, a considerably higher voltage can be used. As compared to three-phase technology, several times the power can therefore be transmitted per cable.
Attempts are being made to compensate for, or at least reduce, this disadvantage of three-phase transmission through the use of artificially cooled cables. To this end, external cooling of the cable as well as internal cooling is used, wherein the cable conductor is designed as a hollow conductor. The external cooling generates fewer problems because the coolant (usually water because of its thermal properties) does not come into contact with voltage-carrying parts.
The advantages of cables with forced cooling are suggestive for use for d-c transmission. Here, however, a specific d-c problem is encountered with external cable cooling. For, contrary to a-c, the break-up of the voltage in the insulation of a d-c cable is determined by the ohmic resistance of the insulating material (generally oil-impregnated paper) in the case of d-c. As expected, the highest field strength occurs at the inner edge of the insulation, i.e. at the surface of the cable conductor, since there, due to the geometry, the ohmic resistance per mm of insulation is highest. Now, the ohmic resistance of cable paper is highly dependent on the temperature; relative to room temperature, the paper heated to the usual operating temperature of a cable can have a resistivity lower by orders of magnitude. This leads to the situation that in a fully loaded d-c cable, the field strength conditions are exactly reversed, i.e. the highest field strength now occurs at the outer circumference of the insulation, i.e. at the cold end.
Therefore, external cable cooling increases the temperature gradient across the cable insulation and thereby leads to a further relative increase of the field strength at the outer edge of the insulation as compared to the inner edge. Therefore, narrow limits are set for this technique; i.e. the current-carrying capacity of a d-c cable cannot be increased substantially by external cooling.
Quite in contrast thereto, the current load can be increased substantially with an internally cooled d-c cable because here, the heat flow is directed predominantly inward, i.e. toward the coolant, but not outward through the cable insulation. The above-described undesirable effect of a load-dependent increase of the field strength at the outer edge of the cable insulation is thereby largely avoided.
With internal cooling, the problem, of course, arises that the coolant, for instance water, is raised to the potential of the cable conductor.
In the known case, this problem is circumvented by connecting all devices required for the circulation and the cooling of the water likewise to high-voltage potential; for instance, the heat exchangers are to be installed insulated and ventilation units are driven via insulated shafts. Likewise, the pumps must be driven via insulating shafts or fed by a transformer with mutually insulated windings. The result of such an arrangement is then, of course, that devices for the contact-less transmission of data and control variables between high-voltage and ground potential must be provided.
This is a disadvantage, since maintenance work on the cooling devices is possible only with the cable disconnected, and the control by forced air cooling can be adapted to the temperature conditions only very roughly.
In the meantime, the technology of power transmission with high-tension d-c (HGU) has developed and utilized water-cooled thyristor valves, which has realized, technically reliably and at economically justifiable cost, the bridging of a potential difference of up to 500 kV d-c with deionized water, and dissipation heat to be removed which fully corresponds to a cable section 30 to 50 km long.
By applying the technique known from HGU it is therefore possible to bridge a sufficiently long cable section with service-friendly cooling equipment and control devices which are at ground potential.
With the self-suggesting design of the cable with a hollow conductor, through which the coolant flows in one direction, the cooling water is warmed up by an approximately constant temperature gradient per unit length. This determines of necessity a difference in the absolute temperature of the coolant and therefore, also of the cable between the entrance point and the exit point of the cooling water. This effect now leads, even though attenuated, to the above-described negative influence on the field strength distribution on the cable dielectric as a function of the cable load.
This negative effect can be avoided if the inner hollow conductor for the coolant is divided, as in the known case, by partitions in such a manner that separate canals are created for the outgoing flow and return of the coolant, where the outgoing and the return canals have the same contact area with the cable conductor and thereby the respective mean value (.theta..sub.m) of the temperature of the outgoing (.theta..sub.Z) and the returning (.theta..sub.R) medium remains approximately constant over the entire length of the cable for the same heat supply per unit length, and thereby, also the temperature at the outer edge of the cable conductor remains constant practically over the entire length of the cable section.
While the thus described cable design assures the same temperature over the entire cable length, there nevertheless remains a dependence of the surface temperature of the cable conductor on the load because of the temperature rise in the coolant, which is dependent on the heat supplied.